Representation of the grey zone of turbulence in the atmospheric boundary layer

Abstract

Abstract. Numerical weather prediction model forecasts at horizontal grid lengths in the range of 100 to 1 km are now possible. This range of scales is the "grey zone of turbulence". Previous studies, based on large-eddy simulation (LES) analysis from the MésoNH model, showed that some assumptions of some turbulence schemes on boundary-layer structures are not valid. Indeed, boundary-layer thermals are now partly resolved, and the subgrid remaining part of the thermals is possibly largely or completely absent from the model columns. First, some modifications of the equations of the shallow convection scheme have been tested in the MésoNH model and in an idealized version of the operational AROME model at resolutions coarser than 500 m. Secondly, although the turbulence is mainly vertical at mesoscale ( > 2 km resolution), it is isotropic in LES (< 100 m resolution). It has been proved by LES analysis that, in convective boundary layers, the horizontal production of turbulence cannot be neglected at resolutions finer than half of the boundary-layer height. Thus, in the grey zone, fully unidirectional turbulence scheme should become tridirectional around 500 m resolution. At Météo-France, the dynamical turbulence is modelled by a K-gradient in LES as well as at mesoscale in both MésoNH and AROME, which needs mixing lengths in the formulation. Vertical and horizontal mixing lengths have been calculated from LES of neutral and convective cases at resolutions in the grey zone.

Cold‐pool‐driven convective initiation: using causal graph analysis to determine what convection‐permitting models are missing

“…Entrainment or detrainment into or from the downdraught or the cold pool can also be relevant. They are mostly determined by turbulence parametrizations, which show substantial deficiencies even in models with resolutions of a hundreds of metres, often denoted as the grey zone of turbulence parametrization (e.g., Honnert, 2016). Surface fluxes have also been found to have a significant impact on cold pools (Gentine et al, 2016;Grant and van den Heever, 2018) and can be sensitive to the resolution of small-scale surface features.…”

Projected changes in extreme precipitation over Scotland and Northern England using a high-resolution regional climate model

“…Since the introduction of the convection-permitting model for weather forecasting and climate research, many new updates have been introduced to the model to address issues arising from operational and climate applications. However, there are still many challenges that are far from resolved, and they are not limited to the diurnal cycle problem that is mentioned here, for example: greyzone turbulence (Honnert 2016;Prein et al 2015), proneness to errors from driving data (Leduc and Laprise 2009). Finding and addressing these model issues are challenging as observations may also be prone to large errors; hence it is critical to document model and observation biases for the benefit of the wider modelling community.…”

Simulating Real Atmospheric Boundary Layers at Gray-Zone Resolutions: How Do Currently Available Turbulence Parameterizations Perform?

“…Modeling SGS turbulence in the gray zone without missing or double counting turbulent energy is a relatively recent topic of research that still lacks robust and validated solutions [9][10][11][12][13]. For grid spacings larger than the gray-zone upper limit, traditional mesoscale modeling approaches can be used to model the impacts of SGS turbulence on the resolved fields.…”